Strange nullpointer exception error when i use a List

Question

i am using a List of <Friend>, Friend is a class that i have with some info about a person, name, desc, etc...

ok, this is the code on the start of the class:

private List<Friend> friends;

and later... in a method of the class:

Friend a = new Friend("Pablo SáeZ", "Total", "39.68333", "-0.32667", new Date());
friends.add(a);

ok, i have the nullpointerexception on the line friends.add(a);

can someone explain why?

thanks

I've rollbacked your edit. Please press Ask Question button if you have a new question. Don't edit an exising one. — BalusC
– BalusC, Commented Nov 17, 2010 at 23:39

BalusC · Accepted Answer · 2010-11-17 20:17:07Z

10

Because you haven't initialized friends yet. It's still null.

You need to initialize it before using.

friends = new ArrayList<Friend>();

A common place to do this job is the class' constructor.

answered Nov 17, 2010 at 20:17

BalusC

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13 Comments

NullPointerException Over a year ago

No.... it doesn't works.... i put this: friends = new List<Friend>(); eclipse says: cannot instantiate the type List<Friend> lol, 7 guys up the wrong answer

BalusC Over a year ago

Put on your glasses. I said ArrayList, not List. You obviously can't instantiate an interface. I'd suggest to stop temporarily with this all and redo the Java basics by a decent tutorial/book :)

NullPointerException Over a year ago

but i dont wanna use arrayList.... i can't do with List? why ? i think i used List sometimes in the past, some years ago when i study

BalusC Over a year ago

No, it's an interface, not a class. You need to use one of its implementations. ArrayList is the best choice for this particular purpose. Just do List<Friend> friends = new ArrayList<Friend>(). See also the Collections tutorial.

BalusC Over a year ago

I think you've to learn about polymorphism and other basic OOP concepts as well :)

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